Some improvements:
- Reorganized javap-at-point and javap-lookup to use a new
  javap-fetch-class instead of first calling the second.
- Added an option (javap-use-java-mode) to control if java-mode is
  turned on.  It defaults to t.
- Function javap-at-point now searches java.lang package if it
  doesn't find it in the current package.